package com.learn.algorlthm;

/**
 * @classname:
 * @Description:数字序列中某一位数字
 * @author: yfpu(fengpu.yang @ ucarinc.com)
 * @Date: 2018-08-08 15:48
 * 题目描述：
 * 数字以 0123456789101112131415... 的格式序列化到一个字符串中，求这个字符串的第 index 位。
 */
public class Learn44 {

    public static int getDigitAtIndex(int index) {

        if (index < 0)
            return -1;
        int place = 1; //1表示个位，2表示十位， ...
        while (true) {
            int amount = getAmountOfPlace(place);
            int totalAmount = amount * place;
            if (index < totalAmount)
                return getDigitAtIndex(index, place);
            index -= totalAmount;
            place++;
        }
    }


    /**
     * place 位数的数字组成的字符串长度
     * 10, 90, 900, ...
     */
    private static int getAmountOfPlace(int place) {
        if (place == 1)
            return 10;
        return (int) (Math.pow(10, place-1) * 9);
    }

    /**
     * place 位数的起始数字
     * 0, 10, 100, ...
     */
    private static int getBeginNumberOfPlace(int place) {
        if (place == 1)
            return 0;
        return (int) Math.pow(10, place -1);
    }


    /**
     * 在place位数组成的字符串中，第index个数
     */
    private static int getDigitAtIndex(int index, int place) {
        int beginNumber = getBeginNumberOfPlace(place);
        int shiftNumber = index / place;
        String number = (beginNumber + shiftNumber) + "";
        int count = index % place;
        return number.charAt(count) - '0';
    }

}
